If he's doing it & denying it, that's a sign he needs his space. He's not ready to share that part of his life with you, and perhaps he will never be. There's quite a bit i've never let my significant others in on. Kinks, fetishes, imagery, etc that i masturbate to, etc. Sometimes, despite being in a marriage, there are things a man wants and needs to keep private. For whatever reason. Some of us have many sides, not all of which we want to share with any given person. I share lots of things about myself on here I never divulge to anyone i know in real life. And there are lots of reasons for that. That's just the way it is.
On the other hand, i sense a little frustration from you, that you'd like to try more, but that he's been same old, same old for 30 years. There are a few ways to deal with that. One is to suggest what YOU'D like to do or try, and if he is not willing, then see if he'd be willing to have an open relationship so that you can be fulfilled. If your needs are not met, you're not being fulfilled, AND he's not open to doing anything to help improve that situation, cheat. Your choice of what you'd prefer to regret. Having cheated....or not seeking opportunity for your own fulfillment.
If I'm hungry and there's no food in the house, I guess i'll have to go out someplace and get some 😉